Tyler Kost, 18, is accused of sexually assaulting a string of teenage girls over five years, authorities said.

Pinal County prosecutors have added two more charges to the growing list against Tyler Kost, an 18-year-old accused of sexually assaulting a string of teenage girls over five years while he attended Poston Butte High School in the San Tan Valley area.

Kost was arraigned Wednesday on one new count of attempted sexual assault and another of attempted child molestation, according to the Pinal County Attorney's Office.

Prosecutors allege that Kost tried to have sex with a girl younger than 15 between January and April 2012.

The teen has previously pleaded not guilty to the allegations against him. His attorneys declined to comment Wednesday.

With the new charges, Kost is now accused on 30 counts of sex crimes against 13 alleged victims.

Some of the girls were under the age of consent at the time prosecutors said. For them, Kost is charged with a total of three counts of sexual abuse, seven counts of sexual conduct, and one count each of child molestation, attempted sexual abuse and attempted child molestation.

For the girls who were minors, but older than 15, Kost is charged with a total of seven counts of sexual assault, seven counts of sexual abuse and three counts of sexual conduct.

Kost has been held without bond in jail since early May, awaiting trial after Pinal County Attorney Lando Voyles announced the 27 grand jury charges and encouraged victims to report crimes in an ongoing investigation.

Voyles said Kost will stand trial as an adult. He is due to appear next in court next month.

The original indictment accused Kost of three counts of sexual abuse with a minor under the age of 15 and six counts of sexual conduct with a minor under the age of 15. The document also included one count of child molestation and 17 counts of sexual assault, sexual abuse or sexual conduct with a minor over the age of 15.